1. Field of the Invention
The present invention pertains to audio amplifiers. More particularly, the present invention pertains to wide bandwidth, current sharing, mosfet audio power amplifier with multiple feedback loops.
2. The Prior Art
MOSFET transistors have a number of advantages when used in audio power output stages. The complementary push-pull source follower stage such as is typically employed in such applications is essentially a voltage operated current output device with extremely high input impedance, thus requiring very small drive currents in the micro-ampere range. These small drive currents are well within the output capability of low level linear circuits such as monolithic operational amplifiers.
Such MOSFET transistors are extremely stable in the source follower connection because they have a voltage gain of less than unity. Further, they are easily driven to the power supply rails using traditional capacitor bootstrap techniques, and they perform well in releasing from the supply rail during maximum voltage excursion since they have virtually no internal carrier storage time, unlike bipolar devices which have a tendency to "stick" to the rail due to carrier storage.
Device capacitance presents little or no degradation of speed of MOSFET transistors in the linear mode because only very small currents are needed to modulate the gate voltage under dynamic conditions such as required in audio power amplifier applications. This feature is considered to contribute to the extremely low transient intermodulation distortion performance of these devices.
While they have a certain amount of capacitance between the gate, source and drain, which must be charged and discharged by the gate drive mechanism, the impact on device bandwidth is small for two reasons. First, when driving the MOSFET devices in the linear region, only small changes in gate voltage are necessary to command large changes in drain/source current. Thus, only small currents are necessary to cause appropriate gate voltage changes. Second, bandwidth is virtually un-affected because the typical 100 ohm resistor commonly used as a gate isolation device and which forms a low pass filter with the device capacitance, typically has a pole at several megahertz, which is well above any recognized bandwidth of interest with regard to audio applications.
One of the fundamental disadvantages of MOSFET devices is that they exhibit a wide variation in gate threshold voltage among individual devices having the same part number, and a temperature coefficient of gate threshold that does not relate to bi-polar transistors, which makes it difficult to establish a quiescent bias point using traditional techniques.
In traditional class AB operation, which is defined as a conduction angle of 180 degrees for each transistor, provisions are made for the conduction period to overlap a few degrees to eliminate crossover distortion. Typically, large amounts of negative feedback are also employed to minimize this and other forms of distortion. In established topologies, the feedback connection is not sufficient to fully remove this error component from the audio output, which makes a certain amount of conduction overlap imperative. Audio power stages with very large conduction overlap, such as class A amplifiers, are considered to have very low audible distortion but achieve this at the expense of very high power dissipation.
Large conduction overlap represents a power drain on the system, and generates heating of the output devices that must be addressed in heatsink design as well as device selection and sizing, and physical circuit layout. It would be advantageous to design a system that will provide a stable overlap region which is high enough to prevent crossover distortion yet low enough to minimize power dissipation with no input signal applied. Quiescent level power dissipation generates an undesirable temperature rise in the transistor heatsink.
In MOSFET devices there is also a lack of correlation between initial threshold voltage and linear transconductance, which makes them extremely difficult to match by selection of devices from a large batch of supposedly identical devices. This means that when devices are connected in parallel, they have to be matched for at least two different and unrelated parameters: that of the transistor under quiescent conditions, and that of the transistor under load. This is a costly process, and in order to compensate for changes in matching of device characteristics with component aging, rational design would require excessive component de-rating, further raising the cost of a product into which they are designed.
Even with a single pair of complimentary output devices, quiescent bias presents difficulties because bipolar devices, which are convenient to use in driver and voltage gain stages, exhibit a temperature coefficient of -2.2 mv. per degree centigrade as opposed to -5 to -7 mv per degree C for MOSFET devices. Topologies can be devised to address these difficulties, but with virtually all of them, including those presented here, there remains a wide variation in output impedance, or damping factor in the crossover region. This is a significant cause of crossover distortion, which is well understood to be a particularly audible, and thus undesirable, form of power amplifier distortion.
Further, even the driving circuitry itself contributes to the tendency of audio amplifiers to exhibit wide variations in damping factor with changes in power level and frequency. It would be desirable to provide a feedback technique that will alleviate this effect. Such a technique would desirably be applied to amplifiers with other types of transistors in the output stage with equally advantageous effect.
Device manufacturers offer exhaustive descriptions of MOSFET parameters and their variations, as well as in-depth studies and discussions of the device characteristics and their implications. These are invaluable in formulating an understanding of the requirements for an easily biased and stable power output stage, but they do not suggest circuitry that will provide functional solutions for production designs.
Some commercially available audio products do successfully address these problems, although matching of components and/or sensitive circuit adjustments are not eliminated. Using a MOSFET as the biasing element for a single pair of output transistors is an effective technique, but it does not address the question of component variation in topologies where devices are connected in parallel. As a result, MOSFET power amplifiers have been confined to the realm of modest output power, or to expensive, hand built, esoteric audio products intended for a very small segment of the audio market. This leaves access to their many advantages economically out of reach for products intended to be sold at competitive prices.
It is thus an object of the present invention to provide a MOSFET audio amplifier circuit which overcomes some of the shortcomings of the prior art.
Another object of the present invention to provide a MOSFET audio amplifier circuit which is able to utilize the advantageous properties of MOSFET devices to as great an extent as possible.
A further object of the present invention to provide a MOSFET audio amplifier circuit which provides current sharing for output devices and has a high bandwidth.
These and other objects and advantages of the present invention will become apparent from the disclosure herein.